This invention relates to analytical sensors for the detection of bioanalytes in a small volume sample, and methods of making and using the sensors.
Analytical sensors are useful in chemistry and medicine to determine the presence and concentration of a biological analyte. Such sensors are needed, for example, to monitor glucose in diabetic patients and lactate during critical care events.
Currently available technology measures bioanalytes in relatively large sample volumes, e.g., generally requiring 3 microliters or more of blood or other biological fluid. These fluid samples are obtained from a patient, for example, using a needle and syringe, or by lancing a portion of the skin such as the fingertip and xe2x80x9cmilkingxe2x80x9d the area to obtain a useful sample volume. These procedures are inconvenient for the patient, and often painful, particularly when frequent samples are required. Less painful methods for obtaining a sample are known such as lancing the arm or thigh, which have a lower nerve ending density. However, lancing the body in the preferred regions typically produces submicroliter samples of blood, because these regions are not heavily supplied with near-surface capillary vessels.
It would therefore be desirable and very useful to develop a relatively painless, easy to use blood analyte sensor, capable of performing an accurate and sensitive analysis of the concentration of analytes in a small volume of sample.
It would also be desirable to develop methods for manufacturing small volume electrochemical sensors capable of decreasing the errors that arise from the size of the sensor and the sample.
The sensors of the present invention provide a method for the detection and quantification of an analyte in submicroliter samples. In general, the invention includes a method and sensor for analysis of an analyte in a small volume of sample by, for example, coulometry, amperometry and/or potentiometry. A sensor of the invention preferably utilizes a non-leachable or diffusible redox mediator. The sensor also includes a sample chamber to hold the sample in electrolytic contact with the working electrode. In many instances, the sensor also contains a non-leachable or diffusible second electron transfer agent.
In a preferred embodiment, the working electrode faces a counter electrode, forming a measurement zone within the sample chamber, between the two electrodes, that is sized to contain no more than about 1 xcexcL of sample, preferably no more than about 0.5 xcexcL, more preferably no more than about 0.32 xcexcL, still more preferably no more than about 0.25 xcexcL, and most preferably no more than about 0.1 xcexcL of sample.
In one embodiment of the invention, a sensor, configured for insertion into an electronic meter, is provided with a working electrode and a counter electrode, and a conductive insertion monitor which provides electrical contact with the electronic meter if the sensor is properly inserted into the meter. The conductive insertion monitor is configured and arranged to close an electrical circuit when the sensor is properly inserted into the electronic connector.
In another embodiment of the invention, a sensor is provided with a plurality of contacts, each contact having a contact pad, which is a region for connection with an electronic meter. The plurality of contacts and contact pads are disposed on a substrate having a length and a width, and each contact pad has a contact pad width taken parallel to the width of the substrate. The sum of the contact pad widths is greater than the width of the substrate. In a preferred embodiment, six electrical connections are made with six contact pads on the sensor but in a width that is approximately the width of four contact pads. For example, a working electrode, three counter electrodes (e.g., one counter electrode and two indicator electrodes), and two insertion trace connections each have a contact pad; connection can be made to each of these six contact pads in the same width of the contact pads of the working electrode and three counter electrodes.
The present invention also includes an electrical connector, for providing electrical contact between a sensor and an electrical meter or other device. The electrical connector has a plurality of contact structures, each which has a proximal contact end for electrical connection to a sensor contact, and a distal end for electrical connection to the electrical device. In one embodiment, a plurality of first contact structures extend longitudinally parallel from the distal to the proximal end. Additionally, one or more second contract structures extend longitudinally next to the first contact structures, from the distal end past the proximal end of the first contact structures, and angle toward a longitudinal center line of the connector. Contact to the sensor is then made via the proximal contact ends.
Preferably, the electrical connector has at least two second contact structures extending longitudinally past the proximal end of the first contact structures and angling toward the longitudinal center line of the connector. After the angled or bent portion, the proximal contact ends of the second contact structures of one embodiment make electrical contact with a single conductive surface of a sensor, such as a conductive insertion monitor. In another aspect, the first contact structures can be configured and arranged to contact one or more working and/or counter electrodes of a sensor, and the second contact structures are configured and arranged to contact one or more conductive insertion monitors.
The sensors of the present invention can be configured for side-filling or tip-filling. In addition, in some embodiments, the sensor may be part of an integrated sample acquisition and analyte measurement device. The integrated sample acquisition and analyte measurement device can include the sensor and a skin piercing member, so that the device can be used to pierce the skin of a user to cause flow of a fluid sample, such as blood, that can then be collected by the sensor. In at least some embodiments, the fluid sample can be collected without moving the integrated sample acquisition and analyte measurement device.
In one embodiment, the sensor is connected with an electrical device, to provide a processor coupled to the sensor. The processor is configured and arranged to determine, during electrolysis of a sample in the sample chamber, a series of current values. The processor determines a peak current value from the series of current values. After the current values decrease below a threshold fraction of the peak current values, slope values are determined from the current values and represent a linear function of the logarithm of current values over time. The processor determines, from the slope values, an extrapolation slope. From the extrapolated slope and the measured current values, the processor determines an amount of charge needed to electrolyze the sample and, from that amount of charge, the concentration of the analyte in the sample.
One method of forming a sensor, as described above, includes forming at least one working electrode on a first substrate and forming at least one counter or counter/reference electrode on a second substrate. A spacer layer is disposed on either the first or second substrates. The spacer layer defines a chamber into which a sample can be drawn and held when the sensor is completed. A redox mediator and/or second electron transfer agent can be disposed on the first or second substrate in a region that will be exposed within the sample chamber when the sensor is completed. The first and second substrates are then brought together and spaced apart by the spacer layer with the sample chamber providing access to the at least one working electrode and the at least one counter or counter/reference electrode. In some embodiments, the first and second substrates are portions of a single sheet or continuous web of material. The invention includes particularly efficient and reliable methods for the manufacture of these sensors.
One such efficient and reliable method includes providing an adhesive having first and second surfaces covered with first and second release liners and then making detailed cuts through the first release liner and the adhesive but not through the second release liner. These cuts define one or more sample chamber regions. A portion of the first release liner is removed to expose a portion of the first adhesive surface, which leaves a remaining portion of the first release liner over the sample chamber regions. This exposed first adhesive surface is applied to a first substrate having one or more conductive traces disposed thereon. The second release liner is removed together with the adhesive and the first release liner of the sample chamber regions in order to expose the second adhesive surface. The second adhesive surface is then applied to a second substrate having one or more conductive traces disposed thereon. This method forms a sensor having a sample chamber corresponding to one of the sample chamber regions.